Survivors of the massacre of 69 people at a youth camp on a Norwegian island are revisiting the scene today to grieve for their lost friends.

Around 1,000 survivors and relatives are expected on Utoya island, accompanied by police and medical staff, to recall the painful memories of the shooting spree by a right-wing extremist.

Anders Behring Breivik has admitted killing 77 people on July 22 when he detonated a truck bomb in central Oslo followed by the shooting massacre on the island.

An Oslo court yesterday extended Breivik's isolation detention by another four weeks saying it still does not know if he acted alone.

Norwegian Prime Minister Jens Stoltenberg said he too would visit Utoya "to take part in their mourning and be there for them (the survivors).

"I will be there as a friend, as a prime minister," he said.

Norway's General Director of Health Bjoern Inge Larsen said he hoped the visits to the island would help survivors and families of the victims come to terms with the deaths.

"The people going there today ... have a lot of anxiety," Larsen said, as he prepared to accompany survivors onto Utoya. "They were life-threatened on this island four weeks ago in a very traumatising manner, so what we are prepared for is to help them to overcome that anxiety."

In the distance, ferries and a pontoon transported survivors, in bright orange life vests, to the forested island used by the ruling Labour Party for political functions, camping and celebrations.

Per Brekke, logistics chief of the operation, said planning the visits had been a big challenge.

"But, of course, the challenge for each individual to re-enter the island is much bigger," he said.

The heavily guarded island, where 400 health care workers, police and other officials escorted the survivors, was visited yesterday by families and friends of the victims killed in the shooting spree.

Tomorrow a national memorial service is to be held at Oslo Spektrum arena, marking the end of a month of mourning.